1. Field of the Invention
The present invention relates to a method of charging a capacitor for use in measurement of insultation resistance of a capacitor, for determination of the quality or failure rate or the like.
2. Description of the Related Art
Generally, in order to determine the quality or failure rate of a capacitor, there is known a method where a direct current measurement voltage is applied to a capacitor and a leakage current (charge current) of the capacitor is measured after the capacitor has been sufficiently charged, by which the insulation resistance of the capacitor is measured. Naturally, an excellent product is provided with small leakage current.
Conventionally, as a method of measuring insulation resistance of this kind, there is known a measurement system prescribed in JIS (Japanese Industrial Standards)-C 5102. According to this system, a measurement time period of substantially 60 seconds is needed since it is necessary to measure a current value after a capacitor has been sufficiently charged. However, in order to reduce the cost and improve the reliability of electronic devices, improvements in production capacity and quality are required also in an electronic part such as a capacitor or the like and therefore, these requirements cannot be met at all by the conventional measuring method where such a long measurement time period is needed for one capacitor.
For charging a capacitor, a method of continuously applying direct current voltage as well as a method of intermittently applying direct current voltage (Japanese Unexamined Patent Publication No. JP-A-4-254769) are known. This method is suitable in the case where characteristic measurement is performed by using a turntable that is intermittently fed and is capable of performing characteristic measurement continuously in respect of a number of capacitors supplied from a parts feeder. In order to measure insulation resistance by using such a turntable, it is possible to use either a continuous system, in which the insulation resistance is measured in respect of capacitors which have finished charging after passing through a plurality of charging regions one by one, or a batch system where charging and measurement of insulation resistance are simultaneously carried out in respect of a plurality of capacitors by stopping a turntable after supplying a predetermined number of capacitors to the turntable. However, a long period of time is needed for charging in either of these systems and their charging efficiency is not excellent.
However, as a result of an intensive study of intermittent application of direct current voltage to a capacitor, the inventors have found that even intermittent application has an effect similar to that of continuous application under certain conditions. That is, intermittent application provides charging characteristics similar to those of continuous application and charging progresses even if there are moments where voltage application is interrupted, as long as they are short time periods.
FIG. 1 and FIG. 2 are graphs showing accurately measured changes of current value, when direct current voltage is continuously applied to a ceramic capacitor, and when it is intermittently applied, respectively, logarithmic current value being plotted against time. In the case of continuous application, as shown by FIG. 1, a substantially constant large current flows during a very small time period (1) after a time t.sub.0 of starting application of a voltage E.sub.0. However, the current value is rapidly lowered successively during a transient time period (2) and thereafter, the current value is lowered with a linear charging characteristic (3) having an inclination. The linear charging characteristic (3) continues until about 1 to 2 minutes has elapsed since the start of charging.
In the case of intermittent application, as shown by FIG. 2, the characteristics (1),(2) and (3) are initially quite similar to those in the case of continuous application. Thereafter, a voltage E.sub.0 is applied a second time at time t.sub.b after voltage application is interrupted once at time t.sub.a. Although the current value is initially increased rapidly as shown by a curve (4), thereafter, the current value is rapidly lowered and is stabilized to a linear charging characteristic (5). Although the characteristic at the top of the curve (4) is not clearly shown, since the abscissa of FIG. 2 designates logarithmic time, the top portion is actually constituted by a horizontal portion similar to (1) and a transient period similar to (2). Further, it is found that the linear characteristic (5) is on a line extended from the linear charging characteristic (3) of the initial voltage application. Even when the intermittent application of voltage is repeated thereafter, characteristics similar to the above-described curves (4) and (5) are repeated and the current value is stabilized on the line extending from the linear charging characteristics (3) and (5). Incidentally, the value of applied voltage E.sub.0 is kept the same in the continuous application and the intermittent application.
A current value i.sub.3 at a time point t.sub.3 after a constant time period T has elapsed since the start of voltage application, stays the same in both the continuous application and the intermittent application. That is, even when the direct current is intermittently applied, as long as the OFF time (t.sub.a through t.sub.b for example) in the intermittent application is a short time period (for example, several hundreds of milliseconds or shorter), a result similar to continuous charging by the continuous application is provided.
According to an experiment conducted by the inventors, in the case of a capacitor having a capacitance value of 0.01 .mu.F. or higher, when the OFF time of intermittent application is set to 500 m seconds or shorter, a result similar to that in the continuous application is provided.
A study of the above-described charging characteristic reveals the following fact. That is, the equivalent circuit of the capacitor is constituted by a capacitance C.sub.0, an equivalent series resistance r, an insulation resistance R.sub.0 and a dielectric polarized component D as shown by FIG. 3. It has been revealed that the nonlinear charging characteristics (1), (2) and (4) in FIG. 1 and FIG. 2 are attributed to the charging region of the capacitance C.sub.0 whereas the linear charging characteristics (3) and (5) are attributed to the charging region of the dielectric polarized component D.
Further, as mentioned above, when intermittent voltage application is performed, a result similar to that of continuous application is obtained and in this case, the charging rate of the capacitor remains unchanged.
Now, as a result of a further study on the intermittent application of direct current voltage to a capacitor, the inventors have found that a capacitor can be charged at a higher speed than in the case where direct current voltage is continuously applied, as long as certain conditions are satisfied. Based on these findings, the charging of a capacitor can be finished at a higher speed than in the case where the voltage is applied continuously, and the measurement of insulation resistance or determination of the quality or failure of the capacitor can be finished in a shorter period of time.
The present invention has been carried out based on the above-described knowledge and it is an advantage of the present invention to provide a method of charging a capacitor which is capable of charging at a high speed by intermittently applying direct current voltage to a capacitor.